Vapor discharge lamp



Aug. 16, 1955 G. MEISTER I2,715,691

VAPOR DISCHARGE LAMP Filed June 29, 1951 ATTORNEY United States Patent OvAPoR DISCHARGE LAMP George Meister, Newark, N. J., assignor toWestinghouse Electric Corporation, East Pittsburgh, Pa., a corpora tionof Pennsylvania Application June 29, 1951, Serial No. 234,395

1 Claim. (Cl. 313-25) This invention relates to lamps and, moreparticularly, to improved metal vapor discharge lamps.

In metal vapor discharge lamps, particularly those containing alkalimetals, which operate at relatively high temperatures, that is, above200 C., the making of metal glass seals which are impervious to attackby the vaporized metal has presented a real problem. The metal glassseals in such tubes usually consist of a refractory metal leading-inconductor, suitably tungsten, and have. an oxide layer at themetal-glass interface.` This oxide layer fails to withstand the attackof the reducing metal vapor, such as cesium vapor, at about 200 C.

Some improvement has been obtained through the use of molybdenumleading-in conductors in lamps which operate up to approximately 250 C.However,y even these molybdenum glass seals fail above 280 C. when thecesium vapor attains suicient pressure to, force its way into contactwith the protective intermediate layer of oxide in the metal glass seal,thereby combining with the oxygen of said layer to form, in the case ofcesium, cesium oxide within the tube and to produce a leakage path alongthe leading-in conductors.

Hence, it has been found advantageous,l according to my invention, tomake the metal-glass seal in alkaliY metal vapor lamps, such as cesium,with a degasified borosilicate glass and a refractory metal leading-inconductor which remains metallic and has no intermediate oxide layer atthe interface between the glass and the said conductors which the cesiumvapor can attack. Duringy the sealing of said conductor to saiddegasitied borosilicate glass, I' prevent the formation of theintermediate oxide layer on the glass metal interface by holding therefractory metal leading-in conductor in or at the tip of the reducingpart of an Oxy-hydrogen flame so that no oxide can form or, if such anoxide is formed, it is immediately vaporized from the conductor by thehot llame.

In its general aspect, the present invention has the object ofovercoming the aforementioned, disadvantages of the prior art in metalvapor discharge tubes.

Specifically, an object of the present invention is an alkali metalvapor lamp having a seal between a refractory metal leading-in conductorand a degasified borosilicate glass which will withstand action by saidmetal vapor.

Another and specific object is an alkali metal vapor lamp having a sealbetween a refractory metal leading-in conductor and a degasifiedborosilicater glass without formation of an intermediate oxide layer atthe glass interface.

An additional object is a method of sealing a refractory metalleading-in conductor to a degasified borosilicate glass of an alkalimetal vapor lamp comprising heating said conductor in or at the tip ofthe reducing part of an Oxy-hydrogen ame so that no oxide forms at theglass metal interface, or if said' oxide is formed thereat, it isimmediately vaporized by the hot llame during the sealing of saidconductor to said glass.

Another object of the invention is a metal vapor lamp 2,7 1 5 ,6 91Patented Aug. 16, 1955.

ICC

operating at temperatures at which said metal vapor will reduce anyoxide of refractory metal used in effecting a seal and having a sealbetween a refractory metal leadingin conductor and a degasiiedborosilicate glass which is impervious to reduction by the metal vaporand resultant early life leakage failures through the seal along theconductor.

Other objects and advantages will appear to those skilled in the art towhich it appertains as the description thereof proceeds, both by directrecitation thereof, and by implication from the context.

Referring to the accompanying drawing, in which like numerals ofreference indicate similar parts throughout:

Fig. 1 is an elevational view of a metal vapor lamp embodying myinvention;

Fig. 2 is an enlarged fragmentary axial sectional View of one end of thelamp of Fig. 1;

Fig. 3 is an elevational view of a seal between a refractory leading-inconductor of the lamp of Fig. 1 and a degasified borosilicate glassbead.

Referring to the drawing in detail, the reference numeral 10 designatesan alkali metal vapor lamp comprising an inner envelope or arc tube 12,an outer evacuated envelope 14 desirably enclosing a gettering device16, and oppositely disposed electrode mounts 18 and 20.

The generally cylindrical arc tube 12 is desirably of alkali-resistantvitreous material, such as glass No. 1, a soda-aluminum borosil'icateglass with a. small percentage of alumina, and is desirably coated onits interior with a. glaze composed largely of boric oxide. This glassis designated as a tungsten sealing glass. It has a softening point at703 C., an annealing point of 496 C., a strain point of 461 C., acoeihcient of expansion of 46 l07 between 0 and 300 C., and a density of2.23. The login of its resistivity at 350' C. is 6.77. lts power factoris .0033' (expressed as a decimal and not asV percent), its dielectricconstant 4.9 andv its loss factor .0161 (expressed as a decimal and notas percent). This glass has the following approximate composition:

This tube 12 contains an inert gaseous filling, such as argon at about20 cm. pressure, for initiating and sustaining a discharge, and a smallquantity of a metal, such as cesium, which is readily vaporizable. Whilea cesium vapor lamp has been selected as an embodiment of thisinvention, it will be understood that other alkali metal vapor lamps orvapor lamps employing metals which readily reduce refractory metaloxides at their operating temperatures are contemplated. To each end oftube 12 is attached a hollow extension 22 of reduced diameter. To theseextensions are axially sealed, as hereinafter explained, refractoryleading-in conductors 23 of electrode mounts 18 and 20.

The electrode mount 18 comprises a fllamentary electrode 24 having anoxide coated helix 25 surrounding a straight return portion 26. It issecured to the inner extremities of conductors 23 as by welding. Nearthe outer portion of one conductor 23 is the gettering means 16 mountedby means of a connector 27 in close proximity to the outer bulb 14. Theouter ends of conductors 23 are connected by flexible, desirablysemicircular connectors 28 to outer leads 30, which in turn are sealedthrough a stem 31 to the outer envelope 14.

The electrode mount 20 differs from mount 18 only in the addition of acup-like anode 32 surrounding the electrode 24 and electrically joinedby connector 34 to one of the leading-in conductors 23. ThisV anode isem- -ployed for direct current operation of the lamp 10. The outerextremities of conductors V23 of mount 20 are joined by flexibleconnectors 34V to outer leads 36. The l leads 36 in turn are secured, asby welding, to hollow `cup-like outer pins 37. These pins 37 are sealedto dishf like projections 35 at one end of the envelope 14. A About eachof the above-mentioned extensions 22 of 1 arc tube 12 are two suitableresilient supporting collars 38, each having a semi-circular bodyr 39clamped to g each other and about the extensions 22k by bolts and jnuts, and spring arm extensions 40 projecting outwardly perpendicularfrom the plane of the body 39 to the inner vWall of outer envelope 14. Aheat resistant tape, such V`Vas'glass tape, is desirably employedbetween said'collars "l 38 and the extensions 22.` i

' YInV outer Yvitreous envelope 14, employed for conserving 7* the heatof operation, there is provided an exhaust tubulation 41 for evacuationpurposes.` A suitable base 42 is axedat the end opposite pins 37'formaking a connection to the outer leads 30 of mount 18. v

According to my invention, theY metal vapor lamp V10 is `provided withan oxide-free seal which Vthe metal f vapor will vnot spoil at operatingtemperatures above 300or C., that is,V between 307 and 325 C., betweenthe leading-in conductors 23 of electrode mounts 18 and 20 and theextensions 22 of arc tube 12.V This approxi# mately 300 C. operatingtemperature corresponds to 2 watts input per cm2 of the surface of arctube 12 when the lamp is operated within an outer euvelope'14.

Y First, the refractory leading-in conductors 23, suitably tungsten ormolybdenum, are chemically cleaned electrolytically to remove any oxidethereon and Vproduce a clean, bright, metallic surface. The cleanVconductors 23 are then heated in or at the tip or inner cone of an l YOxy-hydrogen flame and hermetically sealed to or coated with a'beadV 44of a degasied borosilicate glass, such as glass No. 2, as shown in Fig.3. Y Y This glass No. 2, a soda aluminum borosilicate glass, isdesignated H. R. Clear Chemical. It has Ya softening point at 820i1 10'7between 0 and 300 C., an approximate working pointV of 1220 C. and adensity of 2.23. The login of its resistivity at 350 C. is 6.68. Itspower factor is .0046 (expressed as a decimaland not as percent);its/dielectric constant 4.6 and its loss factorr.021- (expressed as adecimal and not asY percent); Y

.This glass has the following approximate composition:

Per cent SiOz 80 f B203 i3 A12O3- 2 Naz'O-l-KzO 41/2 Miscellaneous Y 1/2The degasilied borosilicate glass No. 2is obtained Vby heating vsaidglass to a temperature of about l700 C. Vto remove the gases therefrom.The intermediate oxide elayer'which normally forms atV the metal glassinterface,

is avoided bythe use of the degasiied borosilicate glass, andthereducing action of the Oxy-hydrogen sealing ame, which preventsformation of the refractory metal oxide on conductors 23, or if theoxide is formed thereon,

immediately'vaporizes said oxidev from said conductors.

YEmploying a suitable jig (not shown) Vaftixed to one Vend of conductors23 the iilamentary electrode24 is mounted on the opposite extremities ofconductors 23. VThese conductors 23, still aligned in the fixture, aresealed together at the beads 44 by a button 46 of dey Y gasitiedboro/silicate glass, such as glass No. 2, the same glass employed forthe beads 44.,

The mount Y18, so formed, is in turn sealedV by means of the button 46to the extension 2.2 of arc tube 12. In

like manner, the mount Y20 is sealed to the oppositelydisposedextension22 of arc tube 12. Y Y Y After a suitable exhaust, which may consist ofa bake, lamentary electrode treatment, evacuation and final gas andmetal ll, the arc tube 12'is tipped off as at 47.

Tne oppositely disposed electrode mounts 18 and 20 are joined byconnectors 28 and 34 to the outer leads 30 and 36 respectively. Thesupport collars 38 are secured about extensions `22 of arc tube 12, andthe outer ening means 16 is vaporized on to the interior wall'of Y'envelope 14 and a base 42 is applied, thereby completing the'lamp 10. YY

' Thus it Will be seen from the foregoing'description that I haveconstructed a metal vapor lamp 10 having a seal between refractory metalleading-in conductors 23 and a degasiiied borosilicate glass bead 44which will withstand metal vapor at operating temperatures of the lampwhere. said metal vapor normally would reduce any oxide of a Yrefractory metal conductor. This seal between therefrractory metalleading-in conductors 23 and the degasiiied borosilicate glass bead 44'ismade without the formation of an intermediate oxide layerV at themetal-glass interface.

In addition, the above-mentioned sealbeing impervious to the metal vaporat the operating temperatures in the lamp 10, a resultant'early leakagefailure through the seal along said conductors 23 are eliminated, aswell'as seals cracked g from overheating. I Y Y Although a preferredembodiment of my invention has been disclosed, it will be understoodthat modifications may be made within the spirit and scope of theappended claim.

I claim: A cesium vapor lamp adapted to'operate at about 2 watts ypersquare centimeter of the inner envelope sur.-V face area to maintainsaid cesium as `a vapor at atemperature at aboutf300 C. and comprisingan inner vitreousV .r light transmitting envelope, oppositely disposedYelectrode Vmounts in said inner envelope and each having rat least'` Ytwo tungsten leading-in conductors'and a lila'mentaryV elec-Vtrode'thereon, an hermetic seal between said conductors Y and saidinner envelope, withA said conductors being oxide-V "j free at leastalong substantially the length of said hermetic Y seal thereon, saidseal having degasified borosilicate 'glass Vhermetically fused directlyto said oxide-free tungsten con-V Y.

ductors without an oxide bond at the metal-glass interfaceY and throughadegasitied borosilicate glass button to said inner envelope, anionizable medium in said inner envelope for initiating and sustaining adischarge admixed' with small quantity of Ycesium which is readilyVaporizable and an outer evacuated vitreous light transmitting enfvelope enclosing said Vinner envelope for conserving the heat ofoperation. Y

References Cited in the le of this patent UNITED VSTATES PATENTSSpinnler Nov. 4, 1952 Y

